Hello，everyone!
I have found that gcc with x32 abi is faster than i686 amd amd64.But some softwares do not support x32 abi.So I want to install a x32 gcc in i686 system.Just the compiler and its depends such as glibc is x32 abi,all other packages will be i686.
How should I do?

I'm not sure I understand precisely what you want, but here are a few pointers:

Some applications are indeed faster when built as 32-bit on a multilib amd64 system, but most are faster as 64-bit (albeit somewhat larger and slower to load). The difference is usually negligibly small, though.

Even if you built an application as 32-bit, the CHOST would still be i686. Using an older CHOST (e.g. i386) is guaranteed to slow the application down significantly (~ 10%, if my memory serves).

Many if not most packages in the amd64 repository are ports from 32-bit code, and so in general work well when built as 32-bit. Doing so is just usually not the point of an amd64 system - if you truly want everything 32-bit, set portage to accept "x86" packages instead.

Assuming that you are correct, and 32-bit is faster, building only the compiler and toolchain as 32-bit would not speed any applications up, only compiles, and even them negligibly, since package build phases usually are not efficient enough to be truly CPU-limited.

In short, if I have understood you correctly, I believe you are mistaken in your beliefs, and recommend that you do nothing at all.